Certain applications of operational amplifiers, in particular of operational power amplifiers, are known in which it is useful to modify the circuit assembly of which the operational amplifier forms part such that the latter is transformed into a buffer, i.e., into an amplifier in which the input circuit is only slightly sensitive to the variations in the load impedance and such that, therefore, it behaves in the manner of a separator between the output and input.
A typical application of this type is the muting control of an audio amplifier. An example of a known circuit assembly for performing this function is shown schematically in FIG. 1 of the appended drawings which shows an operational amplifier consisting of a differential input stage 10, an intermediate gain stage 11 having a compensation capacitance Cc between the input and output, and a power output stage 12. The output terminal 13 of the output stage 12 and the inverting terminal of the input stage 10 are connected to one another by a feedback system consisting of a resistive divider Ra, Rb. A commutator 14 enables the input of the intermediate stage 11 to be switched such that it can be connected selectively to the output of the input stage 10 or to the output of a further differential input stage 15 of which the non-inverting input is connected to a fixed reference potential source VA and the inverting input is connected to the output 13 of the output stage 12. In one position of the switch 14, as shown in this Figure, the circuit assembly behaves in the manner of an operational amplifier with a gain of greater than 1, while, in the other position of the switch 14, it behaves in the manner of a unitary gain amplifier of which the output is independent of the input signal Vin of the operational amplifier and dependent on the predetermined potential VA of the second input stage 15. This solution has the disadvantage of requiring a considerable amount of space since two input stages are required, and also a given degree of complex circuitry since it also has to ensure the stability of the unitary gain amplifier, which entails particular design measures.
Applications are also known in which an operational power amplifier has to be converted into a circuit structure having high impedance between its output terminals, in which the amplifier can be controlled at very low power by a device connected to its output. A structure of this type is useful, for example, in circuits controlling electric motors. A technique for producing a structure of this type consists in deactivating the current generators of the entire amplifier or at least of the output stage. However, in this case the deactivation and subsequent reactivation of the current generators cause considerable variations in the operating conditions, giving rise to intense transient and delay phenomena of the output signal.
A further technique which is sometimes used for attaining high impedance between the output terminals is the provision of a switch in series with the amplifier output; when the switch is opened, the impedance between the output terminals of the amplifier in theory becomes infinite. When this technique is to be used in an integrated circuit, there is a disadvantage in that the switch, through which high currents have to pass, has to have large dimensions and therefore does not lend itself to integration.